A well is drilled into a subterranean formation for enabling access to gas or oil laying therein. Drilling mud is used to facilitate the drilling operation and to hydrostatically suppress the influx of fluid from any pressurized formations encountered. The well is cased and cemented during which the mud is substantially replaced with water. The casing blocks the flow of fluid from the formation. The casing must be perforated to render the well capable of producing fluid from the formation.
Often the well is completed in an underbalanced condition. This involves "swabbing" or "conditioning" the well by pumping out sufficient liquid so that any water remaining in the well will exert insufficient hydrostatic head to restrain the influx of fluid from the formation upon completion. Underbalanced completion is often practised to flush the well of residual drilling mud, water and cuttings.
After perforation, the wellhead is opened to storage and the effluent is inspected over time to ascertain the composition of the well product. This can take some time, often measured in hours, and is always subject to hazards related to the nature of the effluent.
In related prior art, particular characteristics of a well can be determined without prolonged discharge from the well, as disclosed in U.S. Pat. No. 4,123,937 issued in 1978. This reference discloses a method of determining annular gas rates and gas volume in the well annulus by measuring the rate of flow of gas from the well's annulus, then measuring the change in pressure during blocked flow, and then calculating the gas flow as a function of the ratio of the gas flow to the rate of pressure change using mass balance techniques. This prior art method involving alternately open and then restricted annular flow is applied to pumping wells as a means for determining the annulus gas rate without causing a significant change in the bottom hole pressure.
In accordance with the present invention, applicant has developed a process whereby valuable information pertaining to fluid inflow rates and its nature may be determined during the completion of the well, without release of the well contents.